Network storage devices supporting Internet Small Computer System Interface (iSCSI) have gradually come into the recognition of users in the market. In the low to middle end market, a network storage device, usually provided with a physical storage medium, is a device integrated with functions and storage medium. Such a network storage device generally has a capacity below about 10 TB (Tera-Byte). With the increase of user's requirements, a plurality of such network storage devices are needed to form a larger storage space.
To enable an accessing client to utilize the plurality of network storage devices contiguously by using a same access address, the network storage devices need to be virtualized. FIG. 1 illustrates a commonly used structure of a virtualized iSCSI network storage system and the accessing clients thereof in the prior art. As shown in FIG. 1, a storage server is connected to an exchange which connects the accessing clients and network storage devices.
In FIG. 1, a local volume management unit in a network storage device utilizes a local volume to represent a physical storage medium in the network storage device, and provides an access function to the local volume. The local volume may be either a physical volume or a logical volume, depending on the functions of the network storage device and the specific configurations of a user. The storage server performs an in-band virtualization to the network storage devices, manages the local volumes of all of the network storage devices in a centralized manner, and maps the local volumes into a global volume so that the local volumes may be accessed by the clients via one IP address. The global volume is a logical volume. The access to the global volume by an accessing client is done via the storage server. The storage server decomposes the access to the global volume into accesses to the local volumes of one or more network storage devices, and combines the results of the accesses to the local volumes returned from the one or more network storage devices to form a result of the access to the global volume.
However, with the increasingly enlargement of the storage space required and the increase of the network storage devices in the network storage system, the processing capability and connection bandwidth of the storage server may become a bottleneck of the whole network storage system since the storage server alone is responsible for all the read and write operations, thereby decreasing the efficiency of the access to the network storage system.